Depending on the operation modes of liquid crystal, liquid crystal devices are classified into PC (phase change), TN (twisted nematic), STN (super twisted nematic), BTN (bistable twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment), and so forth. They are classified into PM (passive matrix) and AM (active matrix) types depending on driving modes. PM is classified into static, multiplex, etc., and AM is classified into TFT (thin film transistor), MIM (metal insulator metal), etc.
Physical properties of liquid crystal compositions contained in the devices, such as temperature range of the liquid crystal phase, viscosity, optical anisotropy, threshold voltage, specific resistance and elastic constant, are different depending upon the modes. The compositions are prepared by mixing several liquid crystal compounds. In general, the compounds require stability against water, air, heat, and light, broad temperature range in the liquid crystal phase, low viscosity, adequate optical anisotropy, adequate dielectric anisotropy and good compatibility with other compounds. For a nematic phase, a compound having a high upper limit temperature is preferable. For such liquid crystal phases as nematic phase, smectic phase, etc., a compound having a low lower limit temperature is preferable. A compound having low viscosity contributes to creating devices that have a short response time. Adequate optical anisotropy differs depending on the mode of the device. When driving a device with low voltage, a compound having positively large dielectric anisotropy or one having negatively large dielectric anisotropy is preferable. A compound having small dielectric anisotropy is good for adjusting viscosity, etc. In terms of preparing a composition, a compound having good compatibility with other compounds is preferable. Since a device can be used at a temperature lower than the freezing point, a compound having good compatibility at low temperature is preferable.
No liquid crystal device comprising a compound having a chroman ring has been found. For ferroelectric liquid crystal devices, Japanese Patent Publication No. Hei 6-256337 and Japanese Patent Publication No. Hei 6-256339 disclose the compounds represented by formulas (10a) and (10b) below. However, since these compounds do not have sufficiently large positive dielectric anisotropy, they are not ideal for use in liquid crystal devices in such modes as TN, STN, etc.

The prior arts can be found in the references listed below. Better liquid crystal compound, liquid crystal composition and liquid crystal device are desirable.    [Patent reference 1] Japanese Patent Publication No. Hei 6-256337    [Patent reference 2] Japanese Patent Publication No. Hei 6-256339    [Patent reference 3] Japanese Publication No. 2001-316347    [Patent reference 4] Japanese Patent Publication No. Hei 5-25158